Three-year-old Nevaeh Austin was collected from her home about 9am on Wednesday, strapped into the van's second row and taken to Le Smileys Early Learning Centre at Gracemere, near Rockhampton.
The minivan was parked at the centre's front door, but the two staff failed to ensure she left the vehicle.
As temperatures climbed to almost 30C, Nevaeh remained strapped into her seat, clutching her bag.
Almost six hours later, a staff member setting off for the after-school pick-up finally discovered the unconscious preschooler.
She was rushed to Rockhampton Base Hospital in critical condition before being flown to Queensland Children's Hospital in Brisbane, where she remains.
Her devastated family remain at her side, asking for privacy as Nevaeh clings to life and authorities investigate how she had been left in the bus.
"It would appear that Nevaeh was the only child on the bus," Police Detective Inspector Darrin Shadlow said.
"When they have returned to the centre, the driver and one other person who were on the bus at the time, had forgotten that she was there."
The centre's staff were cooperating with the investigation "to a degree", Insp Shadlow said, adding there were clear breaches in safety procedures.
He didn't speculate on potential charges being laid, but vowed to leave no stone unturned in the probe.
Education Minister Grace Grace said her heart went out to Nevaeh and her family, and the toddler should simply had not been left on the bus.
She said state laws changed in 2020, placing obligations on all services transporting children.
"My department, as the Regulatory Authority, is working closely with the QPS to ensure we gather all of the facts surrounding this tragic event," Ms Grace said in a statement.
"The Regulatory Authority does not hesitate to take serious action when services fail to ensure children's health and safety, and where other tragic incidents have occurred, the necessary action has been taken."
Queensland Ambulance Service operations manager Jason Thompson said the situation is distressing for paramedics.
"I get goosebumps just as I speak about it now," he told reporters on Thursday.
"It's a trying time no matter what the situation is, you could put yourself in their shoes (childcare centre staff), they would be very distressed.
"When a child is sick it is traumatic and when one is critical and unconscious, your heart goes out to them."
The incident is similar to the tragic death of Maliq Nicholas Floyd Namok-Malamoo, known as Meeky, in Cairns in February 2020.
Maliq died from heat stress when he was left on a bus for almost six hours after being picked up to go to a childcare centre.
The three-year-old was found still buckled into his seat, and could not be saved.
Maliq's mother, Muriel Namok, said her thoughts are with the Gracemere girl's family and the incident brought back memories of her son's death.
"Just sick, I felt really sick in my stomach. Angry, but definitely sick, I know this feeling all too well," she told Nine's Today program on Friday morning.
Education Queensland introduced its Look Before You Lock policy in October 2021, but Maliq's godmother Rowena Bullio said more needed to be done.
"You look before you lock every time without fail," she said.
"And why it goes on like this? We saw cracks in aged care and then a royal commission came out of that. And this (Maliq) is one child too many, and then we have another child.
"It's too unbelievable. There is a feeling of disbelief, and there is anger now."
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